This article follows the central thread of temperature-driven phase transitions to introduce the sequence of fundamental changes that matter undergoes from low to extremely high temperatures, and connects these transitions to the early evolutionary history of the universe. We first discuss physical processes triggered by increasing temperature, including solid–liquid–gas transitions, plasma formation, the nuclear liquid–gas transition, quark deconfinement, and the electroweak and grand unification theory phase transition. We then reverse the perspective and trace key phase-transition milestones during the cooling of the universe from an extremely hot initial state, including a possible Planck-era transition, inflation, reheating, formation of topological defects, electroweak symmetry breaking, quark confinement, and big bang nucleosynthesis. This article bridges microscopic particle physics and macroscopic cosmology, showing how phase transitions reshape the basic structure of matter and outlining the direction of cosmic evolution.